Terminal-bushing constructions



Oct. 16, 1962 R. E. FRIEDRICH ETAL 3,059,044

TERMINAL-BUSHING CONSTRUCTIONS Filed Dec. 2. 1959 7 Sheets-Sheet 1 o 3 r co w IO Nl AL E INVENTORS Robert E. Friedrich James H. Fro kes mil/QM ATTORNEY Oct. 16, 1962 R. E. FRIEDRICH ETAL 3,059,044

TERMINAL-BUSHING CONSTRUCTIONS Filed Dec. 2, 1959 7 Sheets-Sheet 2 Fig. 2A

Oct. 16, 1962 R. E. FRIEDRICH ETAL 3,059,044

' TERMINAL-BUSHING CONSTRUCTIONS Filed Dec. 2, 1959 7 Sheets-Sheet 5 '7 Sheets-Sheet 4 55 Fig. 3

R. E. FRIEDRICH ETAL TERMINAL-BUSHING 'CONSTRUCTIONS Oct. 16, 1962 Filed Dec. 2, 1959 Removed- During Assembly v he r/ U Bad I W f A u mm 33 n Oct. 16, 1962 R. E. FRIEDRICH ETAL 3,059,044

TERMINAL-BUSHING CONSTRUCTIONS '7 Sheets-Sheet 5 Filed Dec. 2, 1959 7 Filed Dec. 2, 1959 Oct. 16, 1962 R. E. FRIEDRICH ETAL 3,059,044

TERMINAL-BUSHING CONSTRUCTIONS 7 Sheets-Sheei. 6

Oct. 16, 1962 R. E. FRIEDRICH ETAL 3,059,044

TERMINAL-BUSHING CONSTRUCTIONS Filed Dec. 2, 1959 '7 Sheets-Sheet 7 F-l ii l {l l2 1 7 I I l 94 1 l i I l| Fig.7

it; 96 5 i I I Fig.6 1; s

United States Patent 3,959,044 TERMlNAL-BUSHlNG CONSTRUCTIONS Robert E. Friedrich, Baldwin Boro, and James H. Frakes,

Penn Township, Allegheny County, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 2, 1959, Ser. No. 856,775

4 Claims. (Cl. 174-18) This invention relates to terminal-bushing constructions in general, and, more particularly, to a novel type of terminal bushing in which a dielectric gas is substituted for the usual liquid or solid dielectric.

It is a general object of the present invention to provide an improved terminal-bushing construction of compact size and suitable for long trouble-free life with a minimum of maintenance.

A more specific object of the present invention is to provide an improved terminal bushing of the gas-filled type.

Another object of the present invention is to provide an improved terminal-bushing construction adaptable for use with gas-filled electrical apparatus, such as circuitinterrupting apparatus, transformer apparatus, or the like, in which the dielectric gas from the gas-filled electrical apparatus may also be utilized interiorly within the terminal bushing as a dielectric medium therein.

Another object of the present invention is to provide an improved gas-filled terminal bushing in which novel means are provided to obtain a potential tap for the terminal bushing.

Still a further object of the present invention is to avoid the use of paper, oil, compounds etc., generally employed in terminal-bushing constructions, and to employ instead a suitable dielectric gas in a sealed construction, which, preferably, will be noninfiammable, and which will also be self-healing.

A further object of the present invention is to provide an improved gas-filled bushing with good electrical characteristics, which is simple of design, of economical construction, has desirable manufacturing features and which is light in weight.

Yet a further object of the present invention is to provide an improved sealing construction for a gas-filled terminal bushing.

Another object of the present invention is to provide an improved gasketed sealing construction, or a soldersealed sealing construction, suitable for long operational life, and which will require a minimum of maintenance.

Still another object of the present invention is to provide an improved gas-filled terminal bushing adaptable for various current and voltage ratings by suitable dimensioning of the several component parts.

Conventional terminal bushings generally use solid or liquid materials for the insulation to prevent electrical breakdown between the axially extending conductor and the ground flange. The materials generally used are porcelain, paper, plastic, oil, etc. The processing, curing, forming and the manufacturing assembly of these materials are very difficult and costly. In addition, the conventional terminal bushing requires a filling material between the solid insulation and the porcelain shell. A gauge is generally needed to determine the level of the filler, and a chamber is necessary to take care of the filler expansion, as required. Solid insulation bushings are always subject to voids forming or to localized dielectric heating, which may cause permanent damage to the insulation. In the proposed terminal bushing of the present invention, the dielectric gas serves as the insulation medium between the internal conductor and the ground flange to prevent electrical breakdown. A vacuum may be pulled upon the terminal bushing to remove the air, and the insulating gas may be pumped, or released into the terminal bushing from a high-pressure vessel, or the gas may be admitted from an adjacently associated gasfilled electrical apparatus. As a result, no processing, impregnating, curing, etc. are required, and the production assembly time for the terminal bushing is greatly reduced. In addition, the gas itself has an economical advantage over solid insulation material costs, and treating and manufacturing costs.

Accordingly, it is a further object of the present invention to utilize the foregoing advantages in a gas-filled bushing, thereby providing an improved bushing construction.

An ancillary object of the present invention is to provide an improved gas-filled terminal bushing which may be associated with gas-filled electrical apparatus, in which construction the terminal bushing is provided with gas from the gas-filled electrical apparatus, so that upon breakage of any portion of the terminal bushing, the alarm system provided for the gas-filled electrical apparatus will be actuated by the loss of gas pressure from the gas leaking through the bushing, and thereby permit the use of a singl-e alarm apparatus for two diiferent types of gasfilled apparatus, namely the gas-filled enclosure with its electrical apparatus and the associated gas-filled terminal bushing.

Still a further object of the present invention is to provide an improved gas-filled terminal bushing in which the communicating passage for filling the gas-filled terminal bushing is disposed at the upper end of the conducting stud, so that when a heavy dielectric gas, heavier than air, is employed, removal of the dielectric gas from the electrical apparatus will permit the heavy gas within the terminal bushing to remain in place unaffected by the withdrawal of the dielectric gas from the associated gas filled electrical apparatus.

The present invention has particular applicability when employed with gas-filled circuit interrupting structures of the type set forth in US. patent application filed January 23, 1959, Serial No. 788,668, by Russell N. Yeckley, Joseph Sucha and Benjamin P. Baker, and assigned to the assignee of the instant application. In the aforesaid application, there is disclosed a three-pole circuit-interrupting structure comprising metallic tanks filled with sulfur-hexafiuoride (SP gas, and this extinguishing gas serves not only as an interrupting gas, but also as a dielectric gas for insulating the high-voltage parts, disposed interiorly within the metallic tanks, from the grounded tanks. By employing the gas-filled terminal bushings of the present invention with the aforesaid power circuitinterrupting structure, it is possible to permit the same sul-fur-hexailuoride dielectric gas to leak into the terminal bushings by the employment of a gas-communicating hole provided in the terminal stud for the terminal bushing. The resulting structure is highly simplified, and considerably lighter than would be the case should organictype terminal bushings be employed.

A more specific object of the present invention is to provide an improved surface-treating construction for the porcelain shell of a terminal bushing, which is immersed into a sulfur-hexafiuoride gas atmosphere which may be subjected to arcing.

Further objects and advantages will readily become apparent upon reading the following specification, taken in conjunction with the drawings, in which:

FIGURE 1 is a perspective view of a three-pole, gasfilled, power circuit interrupter utilizing terminal bushings of the type proposed by the present invention;

FIGS. 2A and 2B collectively illustrate a partial vertical sectional view taken through the improved terminal FIG. 5 is a top plan view of the terminal bushing of I FIGS. 2A and 2B;

FIG. 6 is a considerably enlarged, vertical sectional, fragmentary view through the potential-tap assembly of the improved terminal bushing;

FIG. 7 is a side elevational view of the potential-tap receptacle assembly associated with the potential-tap assembly of the bushing;

FIG. 8 is a fragmentary sectional illustration of the use of a filter cartridge associated with the lower end of the terminal bushing of FIG. 23;

FIGS. 9 and 10 fragmentarily illustrate, in section,

solder-sealed constructions for the porcelain shells of the terminal bushing, when their use is desired; and

FIG. 11 is a side elevational view of a potential-tap,

,ground plug, which may be used to short the potential .cylinder of the potential-tap assembly.

Referring to the drawings, and more particularly to ;FIG. 1 thereof, the reference numeral 1 generally designates a high-voltage, high-power fluid-blast circuit interrupter, adaptable for the control of a three-phase transmission circuit, whose lines are indicated by the reference characters L L L L L L As noted in FIG. 1,

there is provided an elongated tank structure 2 for each pole-unit of the circuit interrupter 1. The pole-units may generally be designated by the letters A, B and C. The pole-units are identical in construction; consequently, a description of the operation of one pole-unit will suflice for an understanding of the operation of all the pole units.

As'illustrated in FIG. 1, and as more particularly set forth in the aforesaid Yeckley et a1. patent application, a weatherproof enclosing tube 3 encloses an axially-movable operating rod, which mechanically interconnects the three-pole units A, B and C for simultaneous operation, so that there is no relaying difliculty. An acceleratingspring housing 3a is disposed at the end of the stationary weatherproof tube 3, within which is disposed an opening accelerating spring, which biases the enclosed operating rod to the right, as viewed in FIG. 1.

A mechanism and gas-control housing 4 is disposed adjacent to the tank structure 2 of front pole-unit A, ,and not only houses a rather conventional pneumatic mechanism for effecting the axial motion of the enclosed operating shaft within tube 3, but also the housing 4 encloses the equipment employed for maintaining the dual-pressure, gas-supply system at the desired pressures.

Each of the tank structures 2 includes a generallyrhoriz'ontally extending, cylindrical tank portion 5, which has a pair of hinged closure caps 6 at its opposite ends. The hinged closure caps, or heads 6, may be swung to an open position about their hinges, to permit the lateral withdrawal of an arc-extinguishing assemblage from the tank structure 2 during maintenance operations.

Each tank structure 2 is preferably supported by flange plates 8, which may be welded to longitudinally extending steel beams 10, which extend beneath all three poleunits A, B and C. Thus, a crane may be employed to bodily lift the pair of steel beams 19, together with the several tank structures 2 and the housing 4, to permit the bodily placement of the circuit interrupter 1 upon a mounting slab of concrete, or the like, as well known by those skilled in the art.

FIG. 1 shows a pair of upstanding cylindrical steel positioning supports 11, disposed adjacent opposite ends of the centrally situated cylindrical tank portion 5, to permit the mounting of a pair of terminal bushings, gen- 4 erally designated by the reference numerals 12, 13, to the lower internal ends 12A (FIG. 2B) of which is detachably secured the longitudinally extending multi-break arc-extinguishing assemblage, to which reference has been previously made.

Through-type current transformers 14 encircle the terminal bushings 12, 13 of FIG. 1 to provide relaying protection, as is well known by those skilled in the art. There is suflicient clearance between the inner diameter of the current transformers 14 and the weather sheds 12B, 13B

so that the current transformers 14 may be easily placed over the terminal bushings 12, 13 to their mounted operative position.

It will be noted that the mechanism housing 4 has hinged doors 15, opened by handles 16. The circuit interrupter 1, which is of the fluid-blast type, because of its light-weight and portability may be placed upon mounting slabs previously employed for oil circuit breakers of the tank-type for the same voltage and current rating. In addition, the fluid-blast circuit interrupter 1 is of such unusually small size, compared to conventional fluid-blast circuit breakers of the prior art, that its dimensions enable the substitution of it for oil circuit breakers of the same rating.

The longitudinally extending, multi-break arc-extinguishing assemblage extends substantially coaxially along the center of the central cylindrical portion 5 of the tank structure 7.. Reference may be had to the aforesaid application for details of the multi-break arc-extinguishing assemblage, which is secured to the lower interior ends 12A, of the terminal bushings 12, 13 respectively. Generally, the multi-break arc-extinguishing assemblage includes interrupting ability adaptable for the current and voltage rating shown, namely 230 kv., 15,000 mva.

Generally, during an opening operation, the operating mechanism disposed within the mechanism cabinet 4 is such as to effect axial movement of the operating rod disposed within the enclosing tube 3, and, through suitable linkage, to effect opening of the contact structure associated with each of the arc-extinguishing assemblages, not shown, disposed coaxialiy along the central portions 5 of each tank structure 2.. In addition, the mechanism is operable to effect opening of the blast valve associated with a high-potential, gas-storage chamber, which constitutes one end of the arc-extinguishing assemblage. The release of a blast of high-pressure gas from such storage chamber through blast tubes associated with the arc-exwhich the exhaust gas discharges, may be of a considerably lower pressure, say 30 p.s.i.

Suitable compressor equipment is provided within the mechanism compartment 4 to receive the low-pressure exhaust gas and to recompress it, forcing the recompressed high-pressure gas back into an auxiliary highpressure storage tank, not shown, which subsequently feeds such high-pressure gas back into the high-pressure high-voltage storage tank associated with each of the arc-extinguishing assemblages for each pole-unit. Again, reference may be had to the aforesaid Yeckley et al. pattent application for a detailed description of the foregoing features of the circuit-interrupting structure 1.

Preferably the gas disposed within the tank structures 2 is a suitable dielectric gas, which also has considerable arc-extinguishing ability. Such a gas may be sulfur-hexafluoride gas (SP or selenium-hexafluoride gas (SeF or a mixture of one or both of such gases with air, carbon dioxide, helium, argon, or nitrogen. Other gases conceivably may be employed providing they have the desired dielectric and arc-extinguishing characteristics.

The present invention is particularly concerned with the construction of the terminal-bushing structures 12, 13, and their constructional features constitute, partially, the subject matter of the present invention.

With reference to FIGS. 2A and 2B, it will be apparent that the gas filled terminal bushing 12 is of extremely simple construction, and also involves relatively few parts in comparison with conventional terminal bushings, which employ solid or liquid dielectric associated with the bushing.

Extending longitudinally centrally through the center of the terminal bushing 12 is a hollow tubular terminal lead 17. Threadedly secured, as at 18, to the lower interior end of the terminal lead 17, is a bottom terminalplate portion 19, provided with a gasket and supporting a lower porcelain shell, or enclosure 21. As shown, the lower porcelain shell 21 is provided with a plurality of sheds, or skirts 21A to increase the creepage distance axially along the terminal bushing 12.

The upper end of the lower porcelain shell 21 abuts a second gasket 22, which, in turn, is forced under pressure against a plate-like supporting flange 23 at ground potential. Secured, as by welding, to the upper surface 24 of the grounded support flange 23 are a plurality, in this particular instance four, supporting arms 25, which serve to support, in an operative position, the through-type current transformers 14 shown in FIG. 1 of the drawings.

With reference to FIG. 2B of the drawings, it will be observed that the laterally extending supporting arms 25 are provided with notches 26 to permit the transportation of the terminal bushing 12 by a sling carried by a suitable crane.

Also secured, as by welding, to the grounded supporting flange 23 is an upstanding cylindrical support 27, to the upper end of which is secured, as by welding, an outwardly extending flange ring 28. The flange ring 28 is provided with a gasket 22 disposed within a suitable groove, and the gasket 2.? is arranged to be forced under pressure against the lower side or" the upper porcelain shell 30. As shown, the upper porcelain shell is provided with petticoats, or skirts 123 to increase the creepage distance. Disposed at the upper end of the upper porcelain 30 is another gasket 31, which is disposed within a suitable groove provided in a cap bowl 32.

With reference to FIG. 3, which shows more clearly the construction of the upper cap-bowl assembly 33, and the detailed component parts of the sealing gasket 31, it will be observed that a plurality of compression springs 34 are utilized to exert compressive force through the porcelain shells 21, so and through the grounded flange assembly 35, comprising the flange rings 23, 28 and the upstanding cylindrical support 27. In other words, the interior centrally-disposed terminal lead 1'7 is placed under tension, whereas the porcelain shells, 21, 30 maintain the gaskets 2G, 22, 29 and 31 under compressive force, ensuring thereby a fluid-tight sealed arrangement.

As shown more clearly in FIG. 3 of the drawings, the tubular terminal lead 17 is threadedly secured, as at 36, to a spring-seat and terminal assembly, generally designated by the reference numeral 37, and including a laterally extending flange-plate portion 38, against which the upper ends of the several compression springs 34 seat. To prevent the springs 34 carrying any current, preferably insulating bushings 34a are interposed between the upper ends of the springs 34 and the conducting plate portion 38.

A gas-communication hole 39 constituting a passage means, is provided in the terminal assembly 37, and permits the communication of gas from the region 4%) (FIG. 2B), interiorly of the gas-filled apparatus 1, upwardly through the open lower end 61 of the tubular terminal lead 17, into the interior 41 thereof, through the gascommunication hole 39 (FIG. 3), into the region 42, and downwardly past the flange portion 38 of terminal 37 and into the region 43 between the terminal lead 17 and the porcelain shells 21, 3t).

A plurality of removable spring bolts 45 are employed to effect preco-mpression of the compression springs 34 prior to the assembling of the bushing 12. In other words, the spring-seat and terminal assembly 37 is assembled to the cap bowl 32 in a previous assembly operation. During this previous assembly operation, the spring bolts 45 are employed, one for each compression spring 34, and the tightening of the spring bolts 45 ensures a predetermined precompression of the springs 34. During the assembly of the terminal bushing 12, the terminal lead 17 is screwed to the lower terminal plate 19, lower porcelain shell 21 is mounted, central grounded flange assembly 35 is positioned into place, upper porcelain shell 30, and following cementing of the gaskets 31a- 310 (FIG. 3) into position within suitable grooves provided at the lower end of cap bowl 32, the terminal-bushing bowl 32 and the terminal assembly 37 (which previously has been assembled as a unit), are screwed, as at 36, upon the upper end of threaded hollow terminal lead 17 until the gaskets 31a-31c engage the upper surface 46 (FIG. 3) of the top porcelain shell 30.

At this time, the spring bolts 45 are unscrewed and are not employed as a further constructional part of the terminal bushing 12, as indicated in FIG. 2A of the drawings. The spring bolts 45 are only utilized for the precompression of the springs 34.

Obviously, upon removal of the spring bolts 45, as shown in FIG. 2A, the several compression springs 34 are free to expand and consequently exert the compressive force, previously referred to, along the porcelain shells 21, 3t) and the central flange assembly 35.

To accommodate differential temperature expansion between the central terminal lead 17 and the surrounding porcelain shells 21, 30 together with the central flange assembly 35, there is provided a flexible sealed joint comprising a thin metallic diaphragm 47. The outer peripheral edge 43 of the diaphragm 47 is sealed, as by solder 49 (FIG. 3), within a groove 50, provided at the upper peripheral edge 51 of the spring bowl 32.

The inner peripheral edge 52 of the diaphragm 47 is secured, as by a soldered connection 52a, to a sleeve 53, which is threaded, as at 54 to the upper threaded portion of the upper terminal assembly 37, as shown in FIG. 3. In addition, a reinforcing soldered connection 53a is superimposed upon the threaded connection at 54 to ensure a long-lasting fluid-tight seal. Finally, a plate-like back-up shield 56 is screwed, as at 57, onto the threaded end 55 of the terminal member 37. The back-up shield 56 ensures that the diaphragm 47 will not unduly expand during high-pressure conditions existing within the region 43 internally of the bushing structure 12.

As mentioned previously, the terminal bushing 12 may receive its gaseous dielectric from the gas-filled electrical apparatus 1, with which it is employed. As an alternate arrangement, however, the terminal bushing 12 may be filled, through a plugged opening 58 provided at the lower end of the lower terminal 19, and a suitable plug 59 (FIG. 2B) may be employed, screwed into a lower threaded end 60 of the hollow terminal lead 17 With this latter construction, the gas-filled terminal bushing 12 constitutes a separate piece of equipment, which may be pneumatically isolated from the equipment, with which it is associated. Loss of gas from the associated gas-filled apparatus will. not, under such circumstances, affect the integrity of the terminal bushing 12, since it is a totally sealed item.

As another alternative construction, one, or a plurality of plugged bushings 12, could be connected by a pipe connection 23a, through a valve 23b from atank of filling gas 44, as indicated in FIG. 2B.

The flange plate 23 may have a bored passage 23c registering with the pipe connection 23a. Branch connections 23d could lead to other similar gas-filled bushings 12. When the bushing 12 is so operated as a separate item, a pressure gauge 9 may be employed to measure the pres sure internally of the bushing, as indicated in FIG. 2A.

Since the flange plate 23 is at ground potential there is no danger of the storage tank 44 collecting a charge and the valve 23b may be safely operated by station personnel.

In the particular application of the gas-filled terminal bushing 12, where it is desired to associate it with a gasfilled electrical apparatus, and to take the gas from the electrical apparatus into the terminal bushing 12 by the open connection 61 (FIG. 2B) at the lower end of the terminal lead 17, it may be desired to associate with the lower interior end of the terminal stud 17 an activated alumina cartridge 63, more clearly shown in FIG. 8 of the drawings. Preferably the cartridge 63 includes a tubular enclosure, such as a tube 64, inserted as by a press fit or a threaded connection at 65 within the lower end of the terminal stud 17. Activated alumina powder 62 may be maintained within the cartridge '63 by employing suitable end screens 66, closing the ends of the metallic tube 64. This has the advantage that the gas, which is used interiorly of the terminal bushing 12 and is withdrawn from the associated gas-filled electrical apparatus is clean, dry and of high dielectric strength.

When the terminal bushing 12 is associated with circuit-interrupting structure, such as the circuit interrupter 1 of FIG. 1, and it is desired to prevent the entrance of arced gas into the terminal bushing 12, the activated alurnina cartridge 63 has the additional advantage of absorbing any arced products of decomposition from the SP gas. As a result, only clean, filtered gas of high dielectric strength is permitted to enter into the interior 43 of the terminal bushing 12.

FIG. 3 also shows in greater detail the gasket sealing arrangement 31 of FIG. 2A, involving a pair of ringshaped O-gaskets 31a, 31b of resilient material, with a stop gasket 310. The stop gasket 31c prevents the porcelain casing 30 from striking the lower metallic surface 32a of the spring bowl 32. The two resilient O-gaskets 31a, 31b provide double protection, and the leak-detecting bore 67 enables the insertion of a leak detector to determine whether any gas has accumulated past the inner O-ring gasket 31a. Preferably the leak-detecting bore 67 has a plug 68 associated therewith. The leak-detecting bore construction is set forth and claimed in US. patent application, filed November 27, 1959, Serial No. 855,760, by Benjamin P. Baker, and assigned to the assignee of the instant application.

Instead of employing only a single gasket 29 at the lower end of the porcelain casing 30 of FIG. 23, it would, of course, be possible to employ a double O-ring gasketed construction similar to that shown at the upper end of the porcelain casing 30 of FIG. 3.

For certain applications it may be desired to use a solder-sealed construction, such as illustrated in FIGS. 9 and 10 of the drawings. With reference to FIG. 9, it will be noted that an annular sealing ring 69 is employed, which is solder-sealed to the lower end of the porcelain casing 30. The solder-sealed methods set forth in United States Patent 1,852,093 issued April 5, .1932, to Lloyd Smede and Errol B. Shand, and assigned to the assignee of the instant application, may be employed. A flexible ring gasket 70, formed of neoprene, for example, may be employed for sealing, together with a stop gasket 71 to prevent actual contact of the porcelain casing 30 with the flange plate 28. The solder-sealed connection 74 may be employed in conjunction with the lower end of the porcelain shell 30.

A similar arrangement may be employed at the upper end of the porcelain shell 30, as more clearly illustrated in FIG. of the drawings. Here, a metallic sealing ring 76 may be solder-sealed, as at 77, to the upper end of the porcelain casing '30. The outer peripheral edge 78 of the sealing ring 76 may be solder-sealed as at 79 to the outside surface of the cap bowl 32. Again, a stop gasket 84 and a resilient O-ring 75 may be employed, as shown. Packing 72 may be associated with the solder seal 79. Although the aforesaid Smede et a1. patent relates to a fired glazing method for the solder-sealed construction, an ultrasonic process may be employed, such as set forth in US. patent application filed August 13, 1959, Serial No. 833,463 (now abandoned), by Austin Dixon, and assigned to the assignee of the instant application.

As well known by those skilled in the art, it is desirable to obtain a measurement of the voltage present upon the terminal lead 17. As illustrated in FIGS. 6 and 7 of the drawings, a potential-tap assembly, generally designated by the reference numeral 88, is associated with the grounded flange assembly 35 of the terminal bushing 12. A potential cylinder 89, composed of a suitable metallic material, such as aluminum, for instance, is provided interiorly within the cylindrical upstanding support 27. Preferably the potential cylinder 89 is maintained in a proper position by a pair of insulating annular support members 90, 91, which are maintained in position by suitable machine screws 92. As a result, the potential cylinder 89 is electrically isolated from the grounded supporting cylinder 27.

An electrical connection is made to the potential cylinder 89 interiorly of the bushing structure 12 by means of a compression spring 93, which may be formed of any suitable metallic material, such as stainless steel. The contacting compression spring 93 is enclosed within a surrounding insulating tube 94, which is inserted into a bore 95 provided laterally in the ground flange ring 23. Associated with the outer end of the compression spring 93 is a contact 96, which makes electrical connection with a potential-tap receptacle assembly, generally designated by the reference numeral 97, and including an interiorly disposed electrically insulated socket tap, not shown, which may be electrically connected with a potential tap ground plug 98 (FIG. 1 1), when it is not desired to use the potential-tap assembly 88.

In other words, when a pair of terminal bushings 12, 13 are employed with a circuit interrupting tank structure 2, such as illustrated in FIG. 1, only the potentialtap assembly of one of the two terminal bushings is employed, the other being disconnected. When the potentialtap assembly 88 is not used, it is desired to ground the potential cylinder 89 to the grounded flange assembly 35 to prevent it from accumulating a static charge. This is done by using the potential-tap ground plug 98 illustrated in FIG. 11 of the drawings. Usage of the ground contact 96 to the flange asesmbly 35. The potential-tap receptacle assembly 97, which is utilized for voltage measurement, is connected to an external measuring plug, not shown, as Well known by those skilled in the art.

We have discovered that when a porcelain shell with a glazed surface is immersed into a sulfur-hexafluoride gas medium of a circuit interrupter, the arced SP gas will deleteriously affect the glaze. As a result, in the improved terminal bushing 12, when the latter is employed with SP apparatus, the glazing is omitted and a nontracking liquid coating 7 (FIG. 2B) is applied over the unglazed surface of the porcelain shell.

Considering the apparatus set forth in FIG. 1, specifically, the glaze would be omitted from the external surface of the lower, immersed shell 21, and the nontracking liquid coating 7, set forth below, applied. With such treatment, the lower shell 21 is highly resistant to corrosive action of the arced SP gas, and has long operational life.

The activated alumina cartridge 63 will prevent the arced SF,- gas from adversely aifecting the inner surfaces of the bushing, so these surfaces have not been found to require a protective coating.

The applied coating is prepared according to theteachings set forth in United States patent application filed September 15, 1959, Serial No. 840,086, (now abandoned) by Jacob Chottiner, Ben Moreland and Byron V. McBride, entitled Electrically Insulating Coating Composition, and assigned to the assignee of the instant application.

Broadly, the liquid coating compositions which are suitable for use in accordance with this invention consist essentially of (A) from 4% to 36% by volume of at least one resinous polymeric epoxide, (B) from 4% to 36% by volume of a polyamide derived by reacting diethylene triamine with linolenic acid, (C) from 2% to 48% by volume of finely divided aluminum oxide trihydrate and (D) the balance solvent.

One example of such a coating is Part A: Percent by weight Polymeric epoxide 33.3 Polyamide resin 33.3 Aluminum oxide trihydrate 33.3

Part B: V Xylene 80 Cellosolve The composition is prepared by admixing equal parts by weight of Part A and Part B.

From the foregoing description, it will be apparent that there is provided an improved, simplified type of highvoltage terminal bushing, employing a high-dielectric strength gas, such as sulfur-hexafiuoride gas (SP for the main insulation of the bushing. As has been described, the novel terminal bushing 12 of the present invention consists mainly of three main parts, these being the terminal conductor 17, the shell enclosure, including the two porcelain shells Z1, and the interiorly disposed dielectric gas. The hollow terminal lead 17 extends the full length of the terminal bushing 12. and serves a dual purpose. First, it is a mechanical member, and with the spring-cap construction 33 holds the terminal bushing together under continuous pressure, thereby maintaining constant pressure on the gasketed seals 20, 22, 29 and 31.

Secondly, the terminal stud 17 provides an electrical voltage and current path through the terminal bushing 12. The shell enclosure consists of the two end terminals 19, 37, the two porcelain weather casings 21, 30 and the mounting ground-flange assembly 35. Gaskets, solder and welded joints are used to make the shell enclosure gas-tight. It is very essential for the terminal bushing 12 to be tight and not to leak gas. In connection with the gasketed seals, stop gaskets 31c, 71 are employed to prevent porcelain contact with metal, which could cause fracture to the vitreous porcelain. The resilient gaskets 31a, 31b are confined in their grooves and are compressed to make the gas-tight seal. Soldersealed construction may, when desired, be employed, as shown in FIGS. 9 and 10. In such solder-sealed constructions, a metallic collar 69 or 76 is soldered directly to the porcelain and then to the adjacent metal part. This arrangement gives a hermetically-sealed joint with long service life. 7 v v The dielectric gas fills the space 43- between the internal conductor 17 and the interior walls of the shell enclosure. The gas may be used at atmospheric pressure, or at any pressure above atmospheric pressure,'a s desired, to get the best electrical characteristics.

Conventional bushings use solid or liquid materials for the insulation to prevent electrical breakdown between the conductor 17 and the ground-flange assembly. These materials are porcelain, paper, plastic, oil, etc. The processing, curing, forming and the manufacturing assembly of these materials are very diflicult and costly. In addition, the conventional terminal bushing requires a filling material between the solid insulation and the porcelain shell. A gauge to determine the level of the filler is needed, and a chamber is necessary to take care of the filler expansion, which may result upon temperature cycling.

A distinct disadvantage of conventional terminal bushings is the possibility that voids may be formed or localiZed dielectric heating may result, which will cause permanent damage to the insulation. In the proposed bushing of the present invention, the dielectric gas serves as the 1Q insulation medium between the conductor 17 and the ground-flange assembly 35 to prevent electrical breakdown. A vacuum is pulled upon the terminal bushing 12 to remove the air, and the insulating gas is pumped, or released from a high-pressure vessel into the terminal bushing 12 without any diflicult manufacturing operations. No processing, impregnating, curing, etc. are required, and the production-assembly time for the terminal bushing 12 is greatly reduced. The gas itself also has quite an economical advantage over solid insulation material cost, and treating and manufacturing costs.

The terminal bushing of the present invention can be filled with gas pressure and completely sealed by usage of the plug 59, so as to be a self-sustained unit and operate as a separate piece of apparatus. The terminal bushing 12 may also be used with an opening 61 (FIG. 2B) and be filled and operated from the same gas as used in the gas-filled apparatus, to which the bushing 12 may be applied.

With the gas inside the bushing and the air surrounding the bushing 12 both having a dielectric constant of 1, an external uniform voltage gradient is obtained around the flange. This condition greatly reduces the electrical corona and arcing, which occurs when a solid insulation with a dielectric constant much greater than 1 is used in the bushing. Obtaining a uniform gradient and reducing the electrical disturbance results in a bushing with both internal and external high electrical breakdown.

Considerable test data shows that the terminal bushings 12, 13 of the present invention will meet both '60 cycle and impulse test values corresponding to, for example, 230/ 19 6 kv. circuit breaker rating. Other ratings may, of course, be provided by a suitable dimensioning of the parts. The physical dimensions required to obtain the electrical values are very feasible, and comparable to dimensions for conventional terminal bushings.

In the case of an internal sparkover during over-potential tests, the gas insulation is self-healing and does not require repairing or replacing damaged insulation. Gas does not form voids or have localized hot spots. Neither does the gas deteriorate with age nor develop progressive electrical puncture through the body, or creep over the surface as can occur on bushings with organic materials for the insulation. Gas has a favorable characteristic for ionizing around sharp or rough parts of the electrodes, and forms what is equivalent to a semiconducting smooth radius area that prevents further overstress, that may cause electrical failure or radio influence.

As brought out above, the proposed terminal bushing of the present invention has a ground-flange sleeve 27, which will take the regular bushing current transformer 14. A potential tap can be used with the gas-filled bushing by taking a potential from a cylinder 89 inserted inside the flange sleeve 27 and insulated from the flange.

One important advantage of the gas-filled bushing over the conventional bushings is the absence of paper, oil, compounds, etc., which support combustion in case of fire, and can be a considerable hazard to the electrical apparatus and operating conditions. The materials for the proposed gas filled bushing 12 are preferably noninflammable.

The proposed gas-filled bushing offers a bushing with good electrical characteristics, simple of design, of very economical construction, having desirable manufacturing features and is very light in weight.

It is apparent that the gas-filled terminal bushing of the present invention is not confined to use on gas-filled circuit-interrupting structure, but may also be employed on other types of apparatus, such as power transformers. In the latter case, ifthepower transformer is [gas-filled, the gas from the transformer may be employed to fill the terminal bushing 12, as was the case with the gas-filled circuit-interrupting apparatus of FIG. 1.

By having a small gas communication opening 39, any sudden pressure arising within the tank structures 2 during interrupting conditions will not be transmitted immediately to the porcelain shells 21, 30, which might result in their fracture. The small gas communication hole 39 permits only a gradual transmission of gas flow from the gas-filled apparatus 1 into the interior of the terminal bushing construction.

The provision of a small opening 39 between the terminal bushing 12 and the apparatus 1 chokes and prevents the bushing porcelain from being exposed to im pulses of high pressures when the breaker interrupts heavy currents.

When the terminal lead 17 is hollow with the lower end 61 thereof open, it will be apparent that the alarm system for detecting any loss of pressure within the tank structures 2 will in addition also serve at the same time as an alarm system for loss of pressure within the terminal bushing =12 itself. In other words, should, for example, the upper porcelain 30 fracture to permit some gas leakage, this loss of pressure would withdraw more gas from the tank structure 2 into the terminal bushing 12, and this process would trip the circuit-breaker alarm system. As a result of this construction, therefore, the single alarm system for the circuit-interrupting structure 1 serves not only as an alarm for the tank structures 2, but also for the terminal bushings 12, 13 themselves.

By having the gas communication hole 39 disposed at the upper end of the hollow terminal lead 17, the employment of a heavy gas, such as SP will permit the terminal bushing 12 to be removed from the gas-filled apparatus 2, or from a gas-filled power transformer apparatus, with which it may possibly be associated, and due to the heavy weight of the gas, the gas disposed within the interior 43 of the terminal bushing would not tend to fiow out of the terminal bushing 12 following its removal from the associated gas-filled apparatus. If the opening 39 were disposed at the lower end of the ter' rninal bushing 12, the heavy gas might leak out of the terminal bushing 12 following its removal from the gastilled apparatus. As a result, even though the lower end of the terminal lead 17 is opened, as at 61, nevertheless, when a heavy gas is employed, as mentioned above, the terminal bushing 12 may be removed from the gas-filled apparatus without fear of loss of the gas within the bushing, and entrance of contaminated air, possibly moistureladen.

From the foregoing description, it will be apparent that there is provided an improved gas filled terminalbushing construction, which is light and readily adaptable for use as a separate piece of sealed equipment, or for use with gas-filled electrical apparatus, such as circuit interrupting structures or power-transformer equipment.

In practicing the present invention any good dielectric gas may be employed, such as sulfur-hexafluoride (SP selenium hex-afluoride (SeF or mixtures of either one or both of said gases with nitrogen, air, argon, helium and carbon dioxide.

in addition, such gases as octofluoropropane C F Freon 12 (CCI F a gas consisting of a mixture of Preon (0P with SP or another possible dielectric medium would be trifluoromethyl sulfur pentafluoride (CSF Although there has been shown and described specific structures, it is to be clearly understood that the same were merely for the purpose of illustration, and that changes and modifications may readily be made therein by those skilled in the art without departing from the spirit and scope of the invention.

We claim as our invention:

1. The combination with gas-filled electrical apparatus, such as a circuit interrupting structure, transformer struc ture, or the like and a gas-filled terminal bushing associated with said gas-filled electrical apparatus of gas passage meansinterconnecting the gas-filled electrical apparatus and the gas-filled terminal bushing, whereby the gas-filled terminal bushing obtains its dielectric gas from the gas-filled electrical apparatus through said gas passage means, the conductor stud of the gas-filled terminal bushing lbeing tubular and constituting a part of said gas passage means, and a filter cartridge disposed within said tubular conductor stud to filter the gas entering the gas-filled terminal bushing.

2. A gas-filled terminal bushing including a compression spring assembly disposed adjacent one end thereof, an elongated tubular gas conducting conductor stud secured to the compression spring assembly at one end thereof, a pair of surrounding insulating shells one of which engages said compression spring assembly in abutting fashion, a ground flange assembly interposed between the pair of insulating shells, a terminal plate assembly secured to the other end of said elongated tubular gas conducting conductor stud and engaging the other insulating shell in abutting fashion, said compression spring assembly including a spring plate flange portion having a restricted opening provided therein, which communicates with the space interiorly of the tubular gas conducting conductor stud, the other end of said restricted opening communicating with the space interiorly of the pair of insulating shells whereby gas may fill the interior of the terminal bushing for increasing the dielectric strength therein, and the compression spring assembly subjecting the tubular gas conducting conductor stud to tensile stress and the shells in flange assembly to compressive stress, and said dielectric gaseous medium filling substantially the entire space between the conductor stud and the surrounding insulating shells.

3. A gas-filled terminal bushing construction wherein a pair of insulating shells and an interposed ground flange assembly surround a bare axially extending conductor stud, characterized by the combination of a potential tap assembly including an electrically floating metallic potential cylinder disposed inside of the ground flange assembly yet spaced therefrom, said bare metallic potential cylinder being wholly disposed interiorly of the pair of shells and ground flange assembly, means for fixedly mounting said electrically floating metallic bare potential cylinder into a relatively fixed position comprising a pair of insulating annular support members, and means for securing said pair of insulating annular support members adjacent opposite ends of said interposed ground flange assembly.

4. In combination, gas-filled electrical apparatus, such as circuit-interrupting structure, transformer structure, or the like including a tank filled with dielectric gas, a gas-filled terminal bushing extending through the wall of said tank and secured thereto, a tubular conductor stud extending axially through the gas-filled terminal bushing for carrying electrical current from exteriorly of said tank to the region interiorly thereof, gas-passage means inter-connecting the interior of said gas filled tank and the interior of the gas-filled terminal bushing whereby the gas-filled terminal bushing obtains its dielectric gas from the gas-filled electrical apparatus, and said tubular conductor stud being the sole communicating gas-passage means between the gas-fi1led electrical apparatus and the interior of the gas-filled terminal bushing.

References Cited in the file of this patent UNITED STATES PATENTS Re. 18,189 Austin Sept. 15, 1931 1,703,409 Smith Feb. 26, 1929 1,788,873 Jansson Jan. 13, 1931 1,983,370 'Hillebrand Dec. 4, 1934 2,082,055 Higgins June 1, 1937 2,125,089 Skvortzoif July 26, 1938 2,160,660 Hobart May 30, 1939 2,308,022 Peterson et a1. Jan 12, 1943 2,508,184 Morisuye et al May 16, 1950 (Other references on following page) 13 UNITED STATES PATENTS Wilson Sept. 19, 1950 Wiles Nov. 7, 1950 Renfrew et a1 Mar. 29, 1955 Frakes Sept. 23, 1958 Johnston et a1 Nov. 4, 1958 Frakes et al June 2, 1959 Cronin Dec. 8, 1959 Cronin et al. May 17, 1960 14 FOREIGN PATENTS Sweden Oct. 13, 1925 Great Britain July 6, 1936 Great Britain Feb. 28, 1958 Switzerland Sept. 16, 1947 OTHER REFERENCES Gaseous Insulation for High-Voltage Apparatus, published in General Electric Review, February 1948. 

